Cassandra is the main protagonist of Dragon Age: Blood Mage no Seisen. she is investigating a conspiracy within the Chantry.

The Seekers, including Cassandra, attack a group of blood mages who abducted an elven girl with a unique ability from the Circle of Magi. Avexis has the ability to control beasts, and is to be used in a ritual to control dragons. The Seekers kill most of the blood mages but their leader Frenic shapeshifts into a crow and escapes.

They rescue Avexis, but High Seeker Aldren refuses to return Avexis to the Circle; he wants to keep her in custody claiming she is too important for the blood mages to be returned. Late at night Cassandra finds Byron taking the girl from prison. This confuses Cassandra as Byron taught her everything she knew as a Seeker and acted as a father figure for her. Cassandra refuses to leave and follows Byron into the woods. Byron speaks of a conspiracy within the Chantry and insists they get Avexis to safety, to a friend he can trust. Cassandra follows despite being told otherwise, but they soon find themselves surrounded by a group of blood mages. Byron is killed and the blood mages take Avexis.

Cassandra sees a mage trying to flee the scene and captures him. He claims he is not one of them but Cassandra doesn't believe him. When they are captured by templars the mage whispers that he was the friend Byron spoke of and eventually convinces Cassandra. Cassandra distracts the templars and frees them, deciding that it's up to her to find proof of the conspiracy to clear her name as she is branded a traitor.

The blood mages are one step ahead of them each time, and find the other mages killed in a Circle safe house. They had just returned from a paranoid elven informant Lazarro, whose house is in a cliff up a mountain and can only be reached by climbing. Lazzaro betrays them and the blood mages attack, but not before he's given Cassandra vital information with a knife pressed against his neck.

The blood mages are planning to attack the Divine on the 10 year gathering. They are attacked by an ogre, and outside they are surrounded by ogres and golems controlled by the blood mages. They escape using a vial Lazarro was planning to use on them, creating a flash bomb effect. Cassandra is wounded and can hardly walk, but not long after their escape they are found by the templars led by Knight-Commander Martel. At the edge of a cliff and with seemingly no escape, Cassandra throws her and the mage off the cliff, falling into the river below. The mage, who by now has introduced himself as Regalyan D'Marcall, helps her into a cave where they get shelter and he treats her wound. Slowly the mage wins her trust, after he expresses his deep regrets when she tells him about how blood mages killed her brother.

They fall asleep and when the Knight-Commander comes in and sees they're gone, he communicates with an elven stone with Frenic proving to be the traitor within the Chantry. Cassandra manages to get a hold of the stone but is still too injured to fight Martel properly. Regalyan creates a distraction and collapses the tunnel so they can escape. They go on route to approach the High Seeker by entering a secret passage, but he is killed by Martel when he goes to the Divine with the stone as proof against the Knight-Commander.

Martel frames Cassandra and Regalyan for the murder and they are imprisoned. Cassandra learns before her execution that he co-conspired with Callista, the Grand Cleric of Orlais to unleash dragons on the Divine and the gathered Grand Clerics during the ceremony so that Callista would become the new Divine as the only surviving Grand Cleric. However, Regalyan informed Circle mages by sending a bird right before entering the secret passage way, and the mages come to their aid.

Cassandra duels the Knight-Commander, accusing him of betraying the Chantry, to which he retorts that he is ushering in a new era for the Chantry, 'one where events like Kirkwall will not be tolerated'. Cassandra ultimately wins the fight by severing Martel's sword arm; she spares his life and orders him imprisoned, intending to put him on trial for his crimes, but when the Knight-Commander breaks free and attacks, Cassandra beheads him. When 4 dragons, controlled by Avexis, who is in turn controlled by Frenic, attack, she jumps on the back of the first one, killing it. When she is on the back of the second dragon, Frenic orders for a high dragon to be summoned for the attack. The high dragon goes straight for the Divine, but Cassandra stops the attack by flying a wounded dragon into the high dragon, collapsing a wall and wounding Callista. Cassandra is aided by the Seekers to bring the Divine to safety. She goes on to aid Regalyan and the Circle mages who confront Frenic.

Regalyan manages to get Avexis in his arms and Frenic turns into an abomination to defeat the mages and destroy the Chantry after he kills the Grand Cleric of Orlais as she is no longer useful, explaining he took advantage of her ambition to further his own plans. The magic of the Circle mages doesn't hurt Frenic, but Avexis summons the high dragon to burn Frenic. Cassandra leaps on top of him and stabs Frenic in the skull when he falls down to the square beneath, surviving the fall. She rallies the crowd by shouting "For the Chantry!". The crowd responds in the same way.

After the attack Cassandra is seen in Seeker armor again accompanied by Regalyan who is waiting in a dark hallway. Regalyan tells her she is the most beautiful and bravest woman he has after met. She giggles and tells him this is not the right place or time, but to his surprise gives him a kiss on the cheek.

When the door opens Cassandra and Regalyan walk through a crowd, holding hands with Avexis, up to the Divine. Cassandra is proclaimed Hero of Orlais and Right Hand of the Divine by Justinia V.
After the ceremony, the Divine meets with Cassandra in the cathedral. The Divine insists that the attack on the Gathering was only the beginning; a storm is coming and the Chantry must be prepared. Cassandra asks how she can serve as the Divine places a book in her arms.